Evidence of crystallization by particle attachment in synthetic materials is described in numerous contributions. However, efforts to establish the contribution of the particle attachment mechanism to inorganic crystallization in natural environments have barely begun. Here, we show, for the first time, evidence that confirms oriented particle attachment as a crystal growth mechanism that is relevant in sedimentary environments. In these natural settings, oriented particle attachment operates during the formation of highly anisotropically structured clay minerals, which constitute one of the most extensively distributed groups of minerals in the Earth's crust. High-resolution transmission electron microscopy images show that the clay minerals aggregation process occurs in different manners. Smectites aggregate by semi-oriented attachment, while kaolinite, sepiolite and palygorskite aggregate by oriented attachment.

Recent ex situ observations of crystallization in both natural and synthetic systems indicate that the classical models of nucleation and growth are inaccurate. However, in situ observations that can provide direct evidence for alternative models have been lacking due to the limited temporal and spatial resolution of experimental techniques that can observe dynamic processes in a bulk solution. Here we report results from liquid cell transmission electron microscopy studies of nucleation and growth of Au, CaCO3, and iron oxide nanoparticles. We show how these in situ data can be used to obtain direct evidence for the mechanisms underlying nanoparticle crystallization as well as dynamic information that provide constraints on important energetic parameters not available through ex situ methods.